Well, the cable from the MPPT goes to the battery, so it has a power source at both ends.
So a PV side fuse/MCB should cover the MPPT end.
There should also be a current-limiting device that protects the cable from the fault source at the battery end. Geographically, this should be the battery end of the MPPT cable.
With multiple MPPTs I’d prefer protection on each MPPT cable. This is because I prefer protection to have the maximum granularity. ( Especially with fuses).
The relative size of the battery cabling to the MPPT cabling may also mean that fuses have to be individually sized.
Thing is… I’m not sure there is an internal fuse in all MPPTs, or that you can necessarily rely on this for your overcurrent protection.
Certainly the smaller models (like the 100/15) has a 20A fuse that is even externally accessible, but if you look at the manual for the 250/100 (the big boy), as well as the 150/35 (more entry level), both clearly states you should have a fuse on the battery side.
The internal fuse is also, as per the documentation, meant to be used for reverse polarity protection. It “blows the MPPT” (because the non-accessible fuse blows). It is also not sized for whatever cable sizes you are using externally.
So I think the best option remains to either size the main fuse such that all downstream wiring is suitably protected, or individually fuse the MPPT.
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What I was trying to say above. If the system is designed correctly, there is very little value in adding fuses for the MPPT’s
I agree, although I would bicker a little about “correctly”. There is more than one way to do it correctly. In my particular case, the wiring to one of the smaller MPPTs was quite a bit thinner than the rest. The two options on the table was to make the cable thicker or to fuse for the thinner cable. Both are correct answers 
Probably why Victron has made this design possible, to fuse cables correctly.
Edit: My situation, 18 cells, so 62.1v system, makes fuses a challenge. 
As I cannot use commonly available 58v (48v) fuses.
Just thought I put that here, in case someone else ever does the same.
I like the 521A fuse… Where did you find it? 
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If I did that, I would have turned that fuse around … OCD/DVD/PCP/VolKak whatever you want to call that. 
